This invention is directed to hollow, rotationally molded, multi-layered plastic structures and to a method for forming such structures. More particularly, this invention relates to rotationally molded, hollow structures formed from at least two plastic materials having varying physical and chemical properties, one of which is preferably cross-linkable.
The use of plastic in forming relatively large containers which are useful for industrial applications, such as for example, containers for large storage batteries, is known in the art. However, because the use of certain plastics in forming such containers is economically impractical, and because of the poor physical or chemical properties inherent in less expensive plastics, the use of such materials in forming industrial scale structures has not been universally accepted. For example, cases used in manufacturing storage batteries which are installed in telephone exchanges are currently manufactured from thermoset compositions rather than thermoplastic materials. While such a composition is said to have the desired chemical and load resistant properties required for that particular use, cases formed from such hard rubber are quite heavy, are not flame resistant and have poor impact resistance.
Previous attempts to solve the apparent dilemma of balancing cost with required chemical and/or physical properties have included the formation of a multi-layer laminate of plastic materials, which are mechanically or adhesively bonded to each other, one layer having the desired structural strength properties and the other having resistivity to attack by chemicals. An example of such a multi-layered structure may be found in U.S. Pat. No. 3,391,823 to Tijms, which employs a layer of foamed polyethylene between a polyethylene layer and a polyester layer to effect a mechanical bond therebetween. Another example of such a structure is seen in U.S. Pat. No. 3,652,368, to Formo, which discloses a method and a special apparatus for rotationally molding plastic materials in a manner which allows for a blending of the materials at their interface thus forming a mechanically cohesive bond therebetween. Other means of bonding dissimilar plastic materials together have required surface treatments of each plastic layer by methods including flame electrical discharge, chlorination, or chemical oxidation.